‘belongs_to’ and ‘has_one’ Differentiated

One of the more common confusions with ActiveRecord associations is the difference between the `has_one` and `belongs_to` relationships.

However, they are not interchangeable, and there are some serious differences. A `belongs_to` can only go in the class that holds the foreign key whereas `has_one` means that there is a foreign key in another table that references this class. So `has_one` can only go in a class that is referenced by a column in another table.

So this is wrong:

class Transaction < ActiveRecord::Base
  # The transactions table has a order_id  
  has_one :order                
end

class Order < ActiveRecord::Base
  # The orders table has a transaction_id
  has_one :transaction          
end

So is this:

class Transaction < ActiveRecord::Base
  # The transactions table has a order_id
  belongs_to :order             
end

class Order < ActiveRecord::Base
  # The orders table has a transaction_id
  belongs_to :transaction     
end

For a two-way association, you need one of each, and they have to go in the right class. Even for a one-way association, it matters which one you use, and which direction you use it:

class Transaction < ActiveRecord::Base
  # This table has no foreign keys
  has_one :order             
end

class Order < ActiveRecord::Base
  # The orders table has a transaction_id
  belongs_to :transaction     
end

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